1. Field of the Invention
The present invention relates to a production process for a vinyl chloride polymer in which by using an electric wave liquid level gauge for continuously detecting and measuring the liquid level of the polymerization vessel contents, not only during the polymerization step, but also during the raw material addition step and/or the polymer dispersion extraction and washing steps, the state inside the polymerization vessel can be accurately ascertained at all times.
Furthermore, the present invention also relates to a production process for a vinyl chloride polymer in which by also detecting and measuring the liquid level of the polymerization vessel contents during the raw material addition step and/or the polymer dispersion extraction and washing steps, the productivity can be improved with good retention of the product quality.
In addition, the present invention also relates to a production process for a vinyl chloride polymer in which the liquid level of the polymerization vessel contents is observed using the electric wave liquid level gauge, and foaming caused by operation of the reflux condenser is suppressed by addition of an antifoaming agent.
2. Description of the Prior Art
A multitude of production processes for vinyl chloride polymers capable of improving the productivity with no loss in product quality have been proposed, and in some cases adopted, and these processes have used a variety of different approaches. Vinyl chloride polymers are normally produced by batch polymerization. Accordingly, one method of improving the productivity is to shorten the time required for an entire single batch, and particularly the time required for the polymerization reaction step, thereby increasing the number of batches that can be completed within a predetermined unit of time. Furthermore, improvements in productivity can also be achieved by increasing the scale of the polymerization vessel.
Consequently, a reflux condenser is typically used, in addition to a cooling jacket (and where necessary a cooling baffle and/or a cooling coil), to enable the polymerization reaction heat to be removed as efficiently as possible. Removal of heat using a reflux condenser is more economic than heat removal using a cooling jacket, a cooling baffle or a cooling coil, meaning there is a tendency to try and increase the proportion of heat removed by the reflux condenser relative to the total heat removal quantity.
Heat removal by the reflux condenser occurs by condensation of the vaporized monomer. As a result, the pressure of the gas phase within the polymerization vessel decreases, causing an unavoidable foaming phenomenon at the surface of the liquid phase. If the quantity of heat removed by the reflux condenser is increased beyond a certain level, then the generation of scale on the internal walls of the polymerization vessel at the gas-liquid interface becomes increasingly likely, and polymer particles may erupt up inside the reflux condenser with the foam and adhere to the surface of the reflux condenser. In addition, scale generated in this manner can then become mixed with the product during the next polymerization batch, causing an increase in fish eyes. Resolving these problems continues to be the focus of ongoing investigations.
For example, a process has been proposed in which the quantity of foaming is suppressed by using a foam sensor installed within the gas phase portion of the polymerization vessel to detect when the foam at the surface of the liquid phase has reached a predetermined level, and then adding an antifoaming agent at that point (see patent reference 1). In this process, the electrostatic volume type sensor used as the foam sensor is a contact type sensor, meaning the measurement accuracy of the sensor deteriorates if polymer particles become adhered to the sensor section. Furthermore, a multitude of different materials are now being produced as vinyl chloride polymers, and in this process, the optimum level of foaming at which to add the antifoaming agent differs for each material. However, in the process described above the position of the sensor is fixed, meaning only a specific level of foam can be measured. Accordingly, the above process is only capable of detecting the optimum level of foam for one group of polymers.
Furthermore, another process has been proposed in which scale generation and the like is inhibited by controlling the gas-liquid interface within a predetermined range (see patent reference 2). In this process, a polymerization vessel equipped with a liquid level gauge for continuously measuring the liquid level inside the polymerization vessel is used, and the temperature or the flow rate of the cooling water supplied to the reflux condenser is altered in accordance with the liquid level measured by the liquid level gauge. As a result, the quantity of monomer that is refluxed by the reflux condenser can be raised or lowered, enabling the gas-liquid interface to be controlled in a state of continuous surface agitation within a predetermined range, without ever settling. The drawbacks of this process are that the absolute quantity of heat removed is low, and the fact that it is difficult to maximize the heat removal capability of the reflux condenser.
In addition, another process has been proposed in which the heat removal capability of the reflux condenser can be effectively utilized without causing a deterioration in product quality, by selecting the most suitable antifoaming agent and then adding this antifoaming agent at a specific time during operation of the reflux condenser (see patent reference 3). However, this process suffers from the same drawbacks as the process of the patent reference 1. Furthermore in this process, the addition quantity and addition time for the antifoaming agent are determined by inspecting the position of polymer particles adhered to the inside of the vessel following completion of a polymerization batch, and consequently the process is extremely inefficient.
If a non-contact type liquid level gauge is used, then the following types of problems can arise. In order to improve the measurement accuracy, non-contact type liquid level gauges are usually installed with the tip of the sensor protruding into the polymerization vessel from a position on the side wall in the upper region of the polymerization vessel. With this type of configuration, if the quantity of heat removed by the reflux condenser increases too far, then scale can adhere to the tip of the sensor. In order to ensure that foam does not adhere to the sensor tip, the maximum quantity of heat that can be removed by the reflux condenser must be restricted, thus enabling the height of the foam layer generated during the polymerization reaction to be suppressed. As a result, the maximum heat removal capability of the reflux condenser cannot be utilized. In order to prevent scale adhesion to the sensor tip, an indentation can be formed in the side wall of the upper region of the polymerization vessel, and the sensor tip then housed inside this indentation, but this causes the sensor to also pick up microwave reflections off the side walls of the indentation. These reflected microwaves from the side walls cause errors in the measurement of the liquid level, increasing the danger of a marked deterioration in the accuracy of the liquid level measurements.
Patent Reference 1
Japanese Laid-open publication (kokai) No. Hei 4-130103
Patent Reference 2
Japanese Laid-open publication (kokai) No. Hei 7-25909
Patent Reference 3
Japanese Laid-open publication (kokai) No. Hei 9-169805
Batch production of a vinyl chloride polymer is conducted by repeating a batch process comprising essentially the steps of filling the polymerization vessel with an aqueous medium and adding the raw materials such as monomers and the like, conducting the polymerization reaction, extracting the polymer-containing dispersion following completion of the polymerization reaction, and washing the inside surfaces of the polymerization vessel. Managing and controlling the time required for the steps performed prior to and following the polymerization reaction is also an important factor in improving the overall productivity.
Normally, the quantity of the monomer such as the vinyl chloride monomer added is measured by a flow meter installed within the feed line, and is controlled so as to ensure a predetermined constant quantity. However, this flow meter measures only monomer such as the vinyl chloride monomer supplied in a liquid state, and consequently a cavitation phenomenon inside the pump used for feeding and supplying the monomer can cause trouble. Namely, if a portion of the monomer gasifies, then that quantity is not counted by the flow meter, meaning an excessive quantity of the monomer may end up being introduced into the polymerization vessel. In recent years, many feed pumps are being used at levels exceeding their specifications in order to shorten the time required to add the monomer, and this has resulted in a higher frequency of the above type of trouble. Furthermore, after extended usage, the accuracy of flow meters tends to deteriorate, meaning the quantity of the monomer can no longer be accurately measured. The resulting errors in the monomer quantity can have a significant effect on the quality of a product of specific design specifications. In addition, control of the supply of the aqueous medium suffers from similar problems, although the size of the effect may differ. In a conventional production process, detecting those problems which arise from the above types of phenomena caused by the feed pump is extremely difficult, and determining the actual cause of a deterioration in product quality requires considerable investment in terms of time and cost.
Furthermore, shortening the time required for the steps following the completion of the polymerization reaction is also important. Following extraction of the dispersion containing the product polymer from the polymerization vessel, the internal surfaces of the polymerization vessel are washed. Normally the start time for this washing step is determined by measuring the load on the stirring device inside the polymerization vessel. As the liquid level of the reaction mixture inside the polymerization vessel falls the load on the stirring device also falls, and when this load falls below a certain value, operation of the stirring device is halted and the aforementioned washing step is started. However, because the viscosity of the reaction mixture inside the polymerization vessel varies depending on the specifications and the type of the polymer being produced, using the load on the stirring device to determine when the liquid level of the reaction mixture inside the polymerization vessel has fallen below a certain level can be difficult. In other words, even if the washing start time is set on the basis of the load on the stirring device, the height of the residual reaction mixture liquid inside the polymerization vessel may vary. As a result, determining the most appropriate washing start time can be problematic, and in order to ensure that extraction of the reaction mixture has been completed satisfactorily, normally a longer time than is actually necessary is allowed for this extraction, which is not ideal in terms of productivity.